1. Field of the Invention
The present invention relates to a developing agent used to develop an electrostatic charge image and a magnetic latent image in electrophotography, electrostatic printing, magnetography, etc.
2. Description of the Background
In electrophotography, an electric latent image is formed on an electrostatic latent image carrier, developed using a toner to be transferred as a toner image to a transfer material such as paper or the like, and fixed by heating and pressing. With respect to the used toner, not only a past toner of monochromatic black but also a plurality of toners with different colors are used to form an image in a full range of colors.
A toner includes a second-ingredient developing agent used by blending with a carrier particle and a first-ingredient developing agent used as a magnetic or non-magnetic toner. The toner is generally prepared by a melting pulverization method. The melting pulverization method produces a toner particle by melting-mixing a binder resin, a pigment, a mold releasing agent such as wax, a charge controller or the like, cooling down and pulverizing finely, and distributing. An inorganic and/or organic fine particle may be deposited on the surface of the toner particle prepared by the mixing pulverization method as necessary.
The toner particle prepared by the mixing pulverization method generally has an indeterminate shape and a nonuniform composition on the surface. The shape or the surficial composition of the toner particle may vary delicately depending on pulverizing properties of a material or pulverizing conditions, but it is not simple to intentionally control the shape. Further, in the case of a material with high pulverizing properties, the toner particle is pulverized even finely or changed in the shape due to stresses in a developing device. Also, as for the second-ingredient developing agent, a pulverized toner particle sticks to the surface of the carrier so that the developing agent accelerates in charge deterioration. As for the first-ingredient developing agent, the distribution of particles expands so that the pulverized toner particles are scattered or the shape of the toner is changed to lower its developing properties, thereby causing deterioration of image quality.
Meanwhile, in the case of a toner having a mold releasing agent such as wax or the like internally added, pulverization can easily occur on the interface between a binder resin and the mold releasing agent, and thus the mold releasing agent may be exposed on the surface of a toner particle. In particular, when a toner is made of a high elastic resin hard to be pulverized and soft wax such as polyethylene, polyethylene is exposed a lot on the surface of a toner particle. Such a toner is favorable in mold releasing properties or for cleaning an untransferred toner in fixing. However, polyethylene on the surface of the toner particle is separated from the toner particle due to mechanical power such as shearing power or the like in a developing device to easily transfer to a developing roll, a photosensitive body, a carrier, or the like so that they are liable to be contaminated. Thus, such a toner is less reliable for a developing agent.
In consideration of the foregoing problem, JP Patent Publications No. S63-282752 and No. H06-250439, for example, disclose an emulsion polymerization and coagulation method proposed as a production method of a toner where the shape and the surficial composition of a toner particle are intentionally controlled.
In the emulsion polymerization and coagulation method, a resin dispersion prepared by an emulsion polymerization method and a coloring agent dispersion where a coloring agent is disposed in a solvent are mixed to form a coagulated particle corresponding to a toner particle size and fused by heating to obtain a toner particle. According to the emulsion polymerization and coagulation method, the toner may have different shapes from indeterminate to spherical by controlling a heating temperature.
Further, it is tried that the distribution of molecular weight is intentionally controlled in order to improve a fixing property. As a resin with low molecular weight emulsified at low temperature, it can be fixed on a paper with low energy. However, since the resin has low viscoelasticity, an offset phenomenon occurs over a certain level of energy. As a decline of the viscoelasticity at high temperature is eased by using with a resin having high molecular weight, the temperature where an offset phenomenon occurs expands to high temperature. Thus, a mixture of a plurality of resins with different molecular weights or a resin having a plurality of molecular weights by intentionally controlling the distribution of molecular weight of one resin may be used in the method.
In the emulsion polymerization and coagulation method, a toner particle can be obtained by coagulating and fusing at least a fine resin grain dispersion and a coloring agent dispersion. According to the emulsion polymerization and coagulation method, the toner may have various shapes from indeterminate to spherical by controlling a heating temperature.
There is a phase inversion emulsification method where a pigment dispersion or the like is added to a solution containing an organic solvent and water is added thereto.
Further, JP Patent Publication No. H09-311502 discloses a production method of a fine particle by mechanical shearing in an aqueous medium without using an organic solvent.
However, if an additive such as a surfactant or the like adopted for the methods remains in a developing agent over a predetermined amount, it has an ill effect upon charge characteristics.